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Although much of the research into cancer drug resistance has focused on the cancer cells themselves, it is becoming increasingly clear that the tumor microenvironment can significantly affect the success of chemotherapy. The interactions between the tumor cells and their environment can be classified into three main categories: (1) cell-cell contacts, (2) interactions with the extracellular matrix, and (3) interactions with soluble factors/cytokines. Each of these interactions can influence the sensitivity of the tumor cells to treatment-induced apoptosis and can therefore affect the outcome of therapy. The pathways responsible for these effects are just beginning to be elucidated.

The major roles of glutathione (GSH) and glutathione S-transferases (GSTs) in the detoxification of xenobiotics predicts their important role in drug resistance. As such, both GSH and GSTs have been manipulated as targets in the design of novel chemotherapeutic drugs. The discovery that GSTs have additional roles in the cell as regulatory molecules in the mitogen-activated protein kinase pathways together with the more recent discovery of GSH as a regulatory posttranslational modification lend further weight to their already important roles in the anticancer drug resistance response. These findings highlight the importance of these targets in the creation of future novel anticancer drugs. This chapter gives a brief overview of the importance of both GSH and GST in the response to anticancer drug resistance, and highlights some of the anticancer drugs currently being investigated at various stages in the process from lab to clinic.

Metallothioneins (MTs) are a family of proteins that bind some, but not all, heavy metal ions essential for eukaryotic cell function (for example, zinc and copper), and some that are both toxic and not required for cell function (for example, cadmium and mercury). A role for MTs in metabolism and detoxication of heavy metals is strongly suggested by the sensitivity of many MT genes to induction by heavy metals and the ability of MT proteins to bind to many inducing metal ions. However, MT genes are also induced by nonmetal toxins and the expression of MTs varies during normal physiological events (proliferation, differentiation, and cell cycle), suggesting a role or roles not directly related to heavy metal stress. One such role may be the homeostatic regulation of zinc availability. Assessment of function of cells with abrogated MT expression (antisense downregulation of MT and MT gene knockout), increased MT expression by virtue of transient or stable transfection of heterologous MT expression vectors, and in vitro observation of direct and indirect interaction of MT protein with cellular zinc-requiring enzymes and transcription factors has implicated MTs in events modulating resistance to anticancer drug therapy, including zinc-dependent monocyte/ macrophage activation, hormone responsiveness, and transcription factor activity. Evidence exists to suggest that MT (1) regulates immune cell functions by mediating the activity of signal transduction proteins and transcription factors involved in monocyte activation; (2) participates in resisting the effects of damage induced by toxins by regulating the function of the zinc-sensitive transcription factor metal transcription factor 1, the antiapoptotic protein nuclear factor-?B, and the tumor suppressor protein p53, and signaling through the glucocorticoid hormone receptor and other possibly other hormone receptors; and (3) mediates these events in whole or in part by regulating zinc. The importance of inflammation, hormone response, antiapoptotic and zinc-dependent transcription factor function, and zinc regulation in cellular resistance to toxins, coupled with understanding of how MT influences them, sets the stage for rational therapeutic targeting of MT to enhance cancer treatment while sparing normal tissues.

Intrinsic drug or multidrug resistance in previously untreated tumors is often the major obstacle to the success of cancer chemotherapy. Understanding the molecular mechanisms underlying these conditions is a prerequisite to the design of novel strategies aimed at improving current clinical protocols. This chapter focuses on recent experimental evidence concerning two of the features most commonly encountered in multidrug resistant cancer cells: (over)expression of multidrug transporters and disabling of apoptotic pathways.

Drug resistance resulting from the outward efflux of anticancer agents by ATP binding cassette (ABC) transporters such as P-glycoprotein (P-gp) has been well described in vitro in laboratory models. The extent to which multidrug transporters are responsible for clinical drug resistance has been more difficult to determine. In one sense, P-gp can be viewed as a molecular target that was tested in the clinic before there was an adequate understanding of the diseases that were best to study, and before the best inhibitors had been identified. We now recognize that several factors may have impeded the results of clinical trials testing P-gp modulators. First, inhibitors either were not sufficiently potent or required areduction in anticancer drug dose. Alternatively, the presence of other ABC transporters, such as the multidrug resistance-associated protein (MRP1) and the ABC half-transporter ABCG2, may have confounded the results. A single-nucleotide polymorphism (SNP) that limits the expression of P-gp could prevent inhibitor therapy from benefiting patients, and increase toxicity as well. The goal of this chapter is to evaluate new directions in the study of ABC transporters in multidrug resistance, offering fresh approaches to the fundamental question that asks whether ABC transporters are important molecular targets for anticancer drug development.

cis -Diammine-dichloro-platinumII (cisplatin) is an inorganic, square-planar coordination complex that has become one of the most important drugs in the clinical management of cancers over the last three decades. It is similar to classical alkylating agents, in that the central platinum atom interacts covalently with DNA to also form adducts, which are the cytotoxic lesions. When these adducts are detected by damage recognition proteins, signals are transduced that culminate in the activation of apoptosis. However, cisplatin resistance arises when dysregulation of genes reduces the level of adducts formed, reduces recognition of adducts, or inhibits the apoptotic process. Alternatively, prosurvival pathways may become upregulated to increase cell proliferation even when DNA damage is extensive. It is rare to find a single mechanism of resistance within a tumor; in general, several mechanisms coexist to create a complex multifaceted dilemma, which confounds cancer treatment strategies. However, additional platinumbased agents with different spectrums of antitumor activity are entering the clinic, but it is likely that more-potent leads may be identified from among the large reservoir of existing platinum-based agents, provided the multifaceted nature of cisplatin resistance is better appreciated. Although a number of mechanisms have become firmly established in the literature, it appears that several more will be added in time, based on the increasing number of resistance-inducing genes that have been identified from differential gene expression profiles.

There is now a large body of evidence to indicate that the copper (Cu) transporters copper transporter receptor (CTR) 1, ATP7 A, and ATP7B regulate the cellular pharmacology and cytotoxicity of cisplatin (DDP), and that these proteins can mediate acquired DDP resistance. Cells that have acquired resistance to cisplatin demonstrate crossresistance to Cu and vice versa. The crossresistance between DDP and Cu is characterized by parallel changes in Cu and DDP accumulation and altered expression of the Cu efflux transporters ATP7A and ATP7B. Yeast, mouse, and human cells engineered to alter the expression of CTR1, ATP7A or ATP7B exhibit altered sensitivity to both Cu and DDP. Detailed studies of uptake and efflux indicate that each protein can alter the cellular pharmacology of DDP and in some cases, DDP analogs. Immunohistochemical studies of human tumors have identified associations between increased expression of either ATP7A or ATP7B and poor response to treatment with one or another of the platinum drugs. Whereas other transporters may also participate in the influx and efflux of the platinum drugs, available evidence supports the concept that DDP enters the cell, is distributed within the cell, and is exported by mechanisms that have evolved to manage Cu homeostasis.

Resistance to two taxanes, paclitaxel and docetaxel, is frequently observed in cancer patients and limits successful therapy. In experimental systems, resistance to paclitaxel and docetaxel are mediated by alterations in tubulin (the primary site of action of taxanes), proteins that interact with microtubules, energy-dependent efflux pumps, apoptotic proteins, and signal transduction pathways. Clinical correlations with some of these alterations exist, but have not been fully elucidated. Strategies to overcome or circumvent resistance to paclitaxel or docetaxel include inhibition of efflux pumps (which have largely proven to be unsuccessful), the use of novel taxanes or other chemically distinct classes of polymerizing agents that do not interact with drug efflux pumps (currently in clinical trials), and regulation of apoptotic or signal transduction pathways that would restore sensitivity to taxanes. Understanding the basis of resistance at the clinical level is likely to be difficult and complex, but holds the promise of providing a therapeutic opportunity specific to taxane-resistant cancer cells.

Covalent epigenetic modifications such as DNA hypermethylation and histone posttranslational modifications are associated with transcriptional inactivation of many genes and are important during tumor development and progression. Genes involved in key DNA damage response pathways, such as cell cycle control, apoptosis signaling, and DNA repair, can frequently become methylated and epigenetically silenced in tumors. This may lead to differences in intrinsic sensitivity of tumors to chemotherapy, depending on the specific function of the gene inactivated. Furthermore, chemotherapy itself can exert a selective pressure on epigenetically silenced drug sensitivity genes present in subpopulations of cells, leading to acquired chemoresistance. Since the DNA sequences of epigenetically inactivated genes are not mutated but rather subject to reversible modifications via DNA methyltransferases (DNMTs) or histone modification, it is possible to reverse silencing using small molecule inhibitors. Such compounds show antitumor activity and can increase the sensitivity of drug-resistant preclinical tumor models. Clinical trials of epigenetic therapies are now underway. Epigenetic profiling, using DNA methylation and histone analysis, will provide guidance on optimization of these therapies with conventional chemotherapy and will help identify patient populations who may particularly benefit from such approaches.

Delineating mechanisms that mediate de novo and acquired resistance to alkylating agents could potentially lead to novel targets for improving the efficacy of this important class of anticancer drugs. De novo resistance is likely to contribute to minimal residual disease and the subsequent emergence of a more permanent form of drug resistance referred to as acquired drug resistance. The tumor microenvironment represents a rich source of both soluble factors and components of extracellular matrixes, both of which can favor cell survival following drug exposure. Experimental evidence suggests signals that originate from the tumor microenvironment are likely to contribute to de novo resistance and thereby facilitate the emergence of acquired resistance. DNA repair pathways, cell cycle checkpoints, drug metabolism, transporters, and alterations in the apoptotic machinery represent potential mechanisms of resistance to alkylating agents. The role of these pathways in conferring acquired and de novo resistance will be discussed in detail this chapter.